Natural theology

Natural theology is a type of theology that provides arguments for the existence of God based on reason and ordinary experience of nature. It is the science of God insofar as He is accessible to natural reason; insofar as we can infer knowledge about God from the natural order.

History
Since Gottfried Wilhelm Leibniz, natural theology has also been known as the less appropriate name of "theodicy" ("justifying God"), originally meaning "justifying God's existence, given the existence of evil." In the Western religions, the knowledge of God can be classified through revealed theology (knowledge through grace and revelation) and natural theology (using natural reason to come to God). Typical of natural theology have been the traditional philosophical proofs of God's existence, such as the ontological, cosmological, and teleological arguments, as well as Immanuel Kant's less-proved moral argument. Most of the arguments rely on at least one empirical premise.

Ontological argument
It is called the ontological argument because it infers God's existence from what it means to be God, of from the definition of God; it arrives at God's existence purely through rational reflection. There are several different versions, all of which purport to show that it is self-contradictory to believe that there is a greatest possible being. The statement "there is no God" is held to be an absurdity and a self-contradiction. The argument argues that it is a necessary truth that this being exists.

Anselm of Canterbury
The first and most successful author of the argument was Archbishop Anselm of Canterbury. He argued, in 1078, that God is "that than which nothing greater can be thought", and argued that this being must exist in the mind, even in the mind of the person who denies the existence of God. He suggested that, if the greatest possible being exists in the mind, it must also exist in reality. If it only exists in the mind, then an even greater being must be possible - one which exists both in the mind and in reality. Therefore, this greatest possible being must exist in reality, meaning that God exists. To come to any other conclusion is a self-contradiction. The argument was powered by the ideas of having a coherent being that instantiates all perfections, that there is a coherent idea of a being that bestows "great-making" properties, and that existence is a perfection or a "great-making" property. Even in Anselm's lifetime, his argument was challenged (rather poorly) by the monk Gaunilo of Marmoutiers, who came from one of the most distinguished centers of learning in Europe, Marmoutiers Abbey. He believed that Anselm's argument too easily moved from the existence of an idea to the thing that corresponded the idea, claiming that Anselm defined things into existence. Gaunilo worried that one could use Anselm's argument to justify the existence of nonexistent things, such as "an island in which none greater can be conceived." The "perfect island" argument could be easily refuted, as its shore might arbitrarily be increased and decreased, varying with the tide; it is always possible to imagine a bigger physical island, but not any greater thing than God. There is no intrinsic maximum for the size of an island, while there is no greater being than God. Gaunilo did not understand Anselm's point, and Anselm easily rebutted him.

Thomas Aquinas
While Thomas Aquinas believed that God's existence is self-evident, he rejected Anselm's argument. He argued that not everyone who hears the word "God" understand that it signifies a thought, with others seeing God as a body. Since people have different ideas of what God is, there is a problem concerning Anselm's argument.

He had problems with the ontological argument. On his view, even if we assume that everyone shares the same concept of God, it does not therefore follow that he understands what the word "God" signifies exists actually, but only that exists mentally. One natural interpretation of the ambiguous passage is that Aquinas rejected Anselm's second premise, saying that humans have no idea what the sequence of words "A beigng that which none greater can be conceived" really means, saying that God is unlike any other reality known to us.

The success of the argument does not depend on our having a complete understanding of the concept of a being that which none greater can be conceived. While we do not have a complete understanding of the largest number, it can be said that there is no such number. If the concept is coherent, than even a minimal understanding of the concept is sufficient to make the argument. Therefore, Anselm's argument survived both Gaunilo and Aquinas' criticisms.

Immanuel Kant
Immanuel Kant made the most damaging criticism. He directed his criticism at the third premise's claim that a being that exists in both the mind and reality is greater than a being that exists as an idea in the mind. Kant argued that existence does not function as a property, and that "being is evidently not a real predicate... It is merely the positing of something... Logically, it is merely the copula of a judgment." Kant argued that, if God "is", there is no new predicate added to God, instead confirming his conception. What goes wrong with the first version of the argument is the notion of existence being treated as the wrong logical type. Concepts as a logical matter are entirely in terms of logical predicates. Since existence is not a logical predicate, it does not belong to the concept of God. Kant's argument makes a plausible metaphysical point, arguing that existence is not a property in the same way that an apple being red is a property; it is instead a precondition. "Nothing" has no qualities whatsoever, so existence is not a "great-making property", as it is not a property at all, instead being a metaphysical instantiation for the existence of properties. There are two different versions of the ontological argument in Anselm's Proslogium, with the second version not relying on the highly-problematic claim that existence is a property, thereby avoiding the rejections of his argument by Kant. The second version claims that God exists so truly that it cannot be conceived not to exist, that God is a being that which a greater being cannot be conceived, and that necessary existence is a property. A being that necessarily exists in reality is greater than one that does not, meaning that God existing as an idea in the mind but not necessarily in reality, something greater can be conceived. Since there is nothing greater than God, he therefore also exists in reality.